1. Peierls, R. E., Quantum theory of solids (Clarendon Press, Oxford, 1955).
2. De Wolff, P. M., Janssen, T. and Janner, A., Acta Crystallogr., A37, 625 (1981).
3. Janssen, T., Janner, A., Looijenga-Vos, A. and De Wolff, P. M, in International Tables for Crystallography Vol. C, edited by A.J.C. Wilson (Kluwer Academic Publishers, Dordrecht, 1992).
4. Van Smaalen, S., Crystallogr. Reviews, 4, 79 (1995).
5. Withers, R. L. and Wilson, J. A., J. Phys. C19, 4809 (1986).